Photolithography masks are used in the fabrication of integrated circuits and other planar thin film devices, such as surface acoustical wave (SAW) devices. Such fabrication requires the ability to evaporate and/or etch material in precisely defined patterns on a device substrate.
A commonly used technique of pattern definition involves the use of photoresist, an ultraviolet (UV) sensitive material, and photolithography masks. The mask, containing an actual or reverse replica of the desired pattern, is placed over a photoresist covered device substrate. The mask selectively filters out the UV light, thereby limiting UV exposure to predefined photoresist regions. In the case of positive photoresist, the photoresist in the UV exposed areas is softened and easily removed. Actual device formation is then accomplished by either etching and/or evaporating material within areas defined by the remaining photoresist. Several etching and/or evaporation steps are normally used, requiring the alignment of a sequence of masks to a common reference point on the device substrate.
When device formation requires positive photoresist, a dark field mask is generally used. In such case, sequential mask alignment is burdensome as substantially all of the mask is opaque. While a semitransparent mask eliminates this alignment problem, prior techniques for generating semitransparent masks require selective chemical etching of a thin film having the desired optical properties. This etching step precludes the use of semitransparent masks for fabricating devices with small line widths of three micrometers (.mu.m) or less.